HDI Microvia Structure in an RFQ: Stacked or Staggered, and What Else to Declare

A buyer sends out an HDI package. The fabrication notes say: *all microvias laser drilled, via-in-pad permitted*. Three quotes come back and they are 40% apart. Nobody is being dishonest. The three suppliers read that sentence three different ways, priced three different boards, and all three are defensible readings of what was written.

This is the most common failure in fine line hdi circuit procurement, and it is not a design failure. The stackup was fine. The Gerbers were fine. What was missing is that hole structure in HDI is not one field — it is at least four, and leaving any of them open hands the supplier a decision that changes the process route.

PCB168 publishes the outer boundary of what is available on the HDI PCB page: laser drilling down to 0.075mm, layer-to-layer registration of ±25μm, buildups from 1+N+1 through 4+N+4, with blind vias, buried vias, any-layer interconnect and via-in-pad all listed as features. That page tells you the envelope. It does not tell you which point inside the envelope your board occupies. That is your job, and this article is about how to write it down.

The one sentence that costs you comparability

Take the phrase apart. "All microvias laser drilled" fixes the drilling method and nothing else. It does not say:

• whether microvias in different buildup levels sit on top of each other or beside each other
• which target diameter you need, or which of the two published figures you are working to
• whether a via landing in a component pad is filled and capped, or left open
• which layer pairs each blind or buried structure spans

Each of those is a separate cost and yield driver. Each has a cheap answer and an expensive answer. When you leave one blank, a supplier quoting competitively picks the cheap answer, and a supplier quoting carefully picks the expensive one. The spread you see in your quote comparison is not a price difference. It is a specification difference that you created.

That is why the fix is not "ask for a better quote." The fix is to remove the interpretation room before the RFQ goes out.

Field 1: stacked or staggered

In any buildup deeper than one level (1+N+1 has one microvia level per side; 2+N+2 has two, and so on), microvias in successive levels have to either land directly over the level below — stacked — or offset laterally onto a separate landing pad — staggered. Both are normal construction. They are not interchangeable procurement objects.

What changes between them:

Notice the third row, because this is where buyers get surprised. Choosing stacked does not only choose a geometry — it usually drags a fill requirement along with it, whether or not you wrote one. If you specify stacked construction and say nothing about fill, you have implicitly ordered fill, and a supplier who prices without it is quoting a board that cannot be built as drawn.

Write it explicitly, per level if the levels differ:

> Microvia construction: staggered between L1–L2 and L2–L3. Stacked construction is not permitted without written approval.

Or the inverse, if density forces your hand:

> Microvia construction: stacked, L1–L2–L3. Lower-level vias to be filled to provide a planar landing surface.

Either sentence is quotable. "Laser drilled microvia" is not.

Field 2: which diameter, from which published figure

Here is a detail worth being careful about, and it is visible on PCB168's own pages. The HDI PCB page specification table lists laser drilling at 0.075mm. The Key Features block on the same page lists microvia diameter ≥0.1mm. The capabilities page lists minimum drill as 0.1mm mechanical / 0.075mm laser.

These are not contradictory — they are different statements. One describes the drilling process floor, the other describes the microvia feature as typically specified. But if you write "minimum microvia per supplier capability" in an RFQ, you have pointed at two numbers at once, and each quoting engineer will resolve it in whichever direction their process prefers.

So declare a number, not a reference:

> Microvia finished diameter: 0.10mm target, capture pad 0.25mm. Any deviation required for manufacturability to be stated in the written quote with the reason.

Two things this buys you. First, every quote is dimensioning the same hole. Second, if 0.10mm is unnecessarily aggressive for your escape pattern, a supplier can tell you so in writing — and a relaxation you approved is worth far more than one a supplier took silently. Related input fields for the conductor side, which this article does not cover, are in HDI fine line etching RFQ inputs.

Field 3: via-in-pad, filled and capped, or not

Via-in-pad is listed as available. "Available" is not a specification, because via-in-pad describes a location, not a treatment. A via sitting inside a component land can be:

• left open — cheapest, and a solder-void and paste-robbing risk during assembly
• filled with a non-conductive material and planarized — removes the void path, adds process steps
• filled and then copper capped — gives a fully platable, plannable land surface, adds more steps still
• filled and capped with a specified surface flatness requirement over the pad

The differences matter most under fine-pitch BGAs and QFNs, which is exactly where via-in-pad gets used in the first place. And this field has an assembly consequence, not only a fabrication one: if the board goes on to PCB assembly, an unfilled via under a BGA land is an assembly yield conversation you will be having later, with a different supplier, about a board that is already built.

Declare the treatment and the scope of it:

> Via-in-pad under BGA lands U1, U4, U7: filled with non-conductive fill and copper capped, planarized. Via-in-pad elsewhere: not permitted.

Scoping it by component reference is what keeps the cost proportionate. A blanket "all vias filled and capped" is easy to write and is usually paying for treatment on vias that never needed it.

Field 4: blind and buried spans, by layer pair

Blind vias, buried vias and any-layer interconnect are all listed as available on the product page. What a quote needs is not the capability name but the span list. A blind via from L1 to L2 and a blind via from L1 to L4 are different holes with different process implications, and "blind vias as required" tells the supplier neither.

Give a table in the fabrication notes, one row per structure:

Four rows and the ambiguity is gone. Every quoting engineer is now counting the same number of drill and plate operations, and — this is the part that pays off later — your incoming inspection has something to check against. An incoming inspection and acceptance plan can only verify structures that were declared.

What procurement fixes, and what engineering backfills

Not every field belongs to the buyer. Trying to write all of them yourself is how RFQs end up over-constrained and unbuildable. The split that works:

Procurement writes these down and does not leave them open:

• stacked vs staggered, per level
• microvia target diameter and capture pad
• via-in-pad treatment, scoped to named components
• the full span list for every blind, buried and through structure
• which standards govern acceptance — reference IPC-6012, IPC-A-600, IPC-2226 for HDI buildup terminology and IPC-4761 for via protection types by name, and let the supplier state the class and type they are quoting to

Engineering backfills these in the written quote, and you require that they do:

• achievable aspect ratio for your specific span and material combination
• whether your requested diameter is compatible with the chosen fill and cap sequence
• capture pad growth needed to hold registration across your buildup depth
• any construction change required for manufacturability, with the reason stated
• the inspection method used to verify each declared structure — the capabilities page lists per-layer AOI, 100% electrical test, cross-section and X-ray as available methods; which ones apply to your structures is a written-quote answer

The asymmetry is deliberate. Procurement owns what the board must be. Engineering owns what is achievable, and owes you that answer on paper rather than as a silent substitution. This is the same division that governs the material family declaration and the layer count and lamination sequence fields.

The check before you send it

Read your own fabrication notes and ask: could a supplier who wanted to quote low do so without violating a single sentence I wrote? If the answer is yes, find the field that let them, and close it.

Concretely, before release:

1. Every microvia level has a stacked-or-staggered statement.
2. Every structure has a span, a method and a diameter.
3. Via-in-pad treatment is stated and scoped to references, not blanket-applied.
4. Fill and cap requirements are stated wherever stacked construction is specified.
5. Governing standards are named, with class and type left for the supplier to confirm in writing.
6. A line requiring any manufacturability deviation to be stated in the quote, not assumed.

That list is short because hole structure is a bounded problem. It only looks unbounded when it is written as one sentence. If you want the surrounding document set checked at the same time, the HDI quote package checklist covers what travels with these notes.

FAQ

Is staggered always the safer choice?

It is generally more tolerant of registration variation, because the lateral offset means each level is not required to land on the level below. That tolerance costs routing area, which is often the reason stacked was considered in the first place. There is no universal answer — declare the one your escape routing actually needs and let the supplier confirm it is achievable on your buildup in the written quote.

Can I just write "minimum microvia diameter per your capability"?

You can, but you will not get comparable quotes. PCB168 publishes 0.075mm as the laser drilling figure on the HDI PCB page specification table and ≥0.1mm as the microvia figure in the Key Features on the same page. Pointing at "your capability" points at both. Name the diameter you need and let engineering tell you in writing if it should change.

Does specifying stacked microvias automatically require filled vias?

In practice a stacked structure needs a planar surface for the next level to land on, so fill normally comes with it. The important point for an RFQ is that if you specify stacked and stay silent on fill, you have created an implied requirement — and implied requirements are exactly what makes quotes incomparable. State it.

Should via-in-pad be applied to the whole board?

Rarely worth it. The treatment exists to solve a specific problem under fine-pitch lands. Scoping it to named component references keeps the cost attached to the area that needs it. A blanket requirement is simpler to write and usually more expensive than the board needs.

Who decides the aspect ratio?

The supplier, in the written quote. Achievable aspect ratio depends on the span, the material and the plating process, and PCB168 does not publish a figure for it. Ask for it as a quote deliverable rather than specifying one yourself — a number you invent can quietly make your board unbuildable.

How is a declared structure actually verified on delivery?

Through the inspection methods named in the written quote. The capabilities page lists per-layer AOI, 100% electrical test, cross-section and X-ray among available methods. Which of those apply to your specific structures, and the acceptance class under IPC-6012 and IPC-A-600, are confirmed by engineering review in the written quote, not assumed from this page.

Related links

• HDI PCB
• Capabilities
• PCB Assembly